Motion compensation systems are used in the offshore drilling industry to mitigate the undesired effects between floating offshore platforms and assemblies which may be fixed to a floor of a body of water. These undesired effects include changes in forces and stresses on the assemblies and the hysteresis, i.e. the rapid start and stop of vertical movement of the offshore platforms. Currents, wind and other weather phenomena all impact the elevation of the surface of the body of water thereby creating vertical movement between the offshore platforms and the assemblies.
Many motion compensation systems use a cylinder and a gas/liquid accumulator apparatus to dampen the movement between the offshore platform and an assembly which may be fixed to the seabed. In these systems, the cylinder houses a piston with a rod extending from one side of the piston. Pressurized liquid is used on both the rod side of the piston and the opposing side to counterbalance the vertical movement of the offshore platform. A gas/liquid accumulator is used to introduce pressurized liquid to the rod side of the piston. The gas/liquid accumulator typically consists of a vessel with two chambers separated by an elastic diaphragm, a totally enclosed bladder or a floating piston. One chamber contains hydraulic liquid and is connected to a hydraulic line that is in fluid communication with the rod side of the piston. The other chamber contains a pressurized gas. As the pressure of the gas further increases within the gas/liquid accumulator, the hydraulic liquid is forced out of its chamber and into the cylinder on at least the rod side of the piston.
The use of gas/liquid accumulator apparatuses in motion compensation systems is undesirable because such an accumulator adds additional cost and complexity to the operation of the motion compensation system. Depending on the magnitude of the load to be compensated, multiple gas/liquid accumulators may be required to effectively operate the system. This is due in part to the load variance of pressurized liquid. Pressurized gas at the same pressure and volume as that of the pressurized liquid is able to compensate a higher load. Therefore a need has arisen for a motion compensation system that can effectively operate using pressurized gas in lieu of pressurized liquid on the rod side of the cylinder thereby eliminating the use of gas/liquid accumulator systems.